Photovoltaic (PV) power generation is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of cells (called “solar cells” herein) containing a photovoltaic material. Cell materials include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide (CIGS). Due to the growing demand for renewable energy sources, manufacturing of solar cells and photovoltaic arrays has advanced considerably in recent years. Solar cells include thin film solar cells and crystalline solar cells. Thin film solar cells are normally much larger than crystalline cells. Both crystalline solar cells and thin film solar cells include an electrode structure that collects electrons freed by photon impingement on the solar cell. The electrode structure includes finger electrodes and buss electrodes. Generally, the buss electrodes are much larger than the finger electrodes.
Thick film screens made with woven stainless steel wires are conventionally used to form solar cell electrode structures, which are often referred to as prints. A printing process produces the prints by using conductive paste or ink printed through the thick film screens (the conductive material will be called “paste” herein; it is understood that the teachings herein are equally applicable to use of thinner conductive material that may be called “ink”). A typical screen for solar printing is 325 mesh with a 0.9 mils wire diameter. The thick film screen mesh wire may stretch during printing life cycles such that printed images may distort. Thick film screens typically have an open print area of about 42%, with the best screens having an open print area of about 50%. Typical thick film screens have a base wet print thickness of about 1 mil. A common emulsion (used to block the transfer of paste in areas that are not to be printed) is about 0.5 mils thick and requires the wet print thickness to be about 1.5 mils.
As solar cells become larger and finger electrodes become smaller, thick film screens run into some limitations. When the widths of the finger electrodes approach 50 μm to 75 μm (about 2 to 3 mils), the wire diameter of the thick film screens becomes an issue in print line definition. The mesh structure becomes a greater portion of the open line width, leading to sawtooth edges where images formed by the mesh wires intersect with the intended edge. A practical limit for line resolution in the thick film screens is about 3 mils wide. As the area of the printed image gets larger, trampoline screens are required to provide precise mounting and uniform tension. However, trampoline screens are very expensive and prone to distortion during their printing life cycles. Screens can also deform under tension throughout their useful lives, leading to dimensional instability.
There remains a need for developing new screens and processes to resolve issues associated with thick film screens and trampoline screens. A simple wish list for an advanced screening technique is that sawtooth edges should be eliminated, the percentage of open area in printed areas should increase, and the image should be dimensionally stable throughout its useful life.